A high-hardness sintered body mainly composed of cubic boron nitride has conventionally been used in an application such as a cutting tool. Such a sintered body is normally implemented by a composite sintered body containing cubic boron nitride and a binder mainly aiming to maintain strength (improvement in wear resistance) of the former and the like. For example, a sintered body having a sea-island structure, in which cubic boron nitride particles (islands) are dispersed in a continuous phase (sea) of the binder (Japanese Patent Laying-Open No. 53-077811 (Patent Document 1) and Japanese Patent Laying-Open No. 10-182242 (Patent Document 2)), has been known.
Such a sea-island structure has been adopted, aiming to promote a sintered state of cubic boron nitride particles to thereby improve chipping resistance such as toughness, by not allowing contact between the cubic boron nitride particles but separating the cubic boron nitride particles from each other in order to avoid an unsintered state of cubic boron nitride caused at a contact portion where the cubic boron nitride particles physically contact with each other.
On the other hand, as the cubic boron nitride particles do not come in contact with each other but are separate from each other (that is, the structure is such that the cubic boron nitride particles are present discontinuously), such an excellent characteristic as high thermal conductivity specific to cubic boron nitride is not sufficiently exhibited, which leads to problems such as insufficient heat resistance or low toughness. Specifically, in the case that the sintered body is used in a cutting tool, increase in a temperature at a cutting edge cannot be suppressed during a cutting process. Moreover, small cracking that originated in the sintered body develops in the binder having low toughness, and chipping is more likely. Therefore, if the sintered body is employed in the cutting tool used in interrupted cutting and high-efficiency cutting such as heavy cutting and high-speed cutting, satisfactory tool life cannot sufficiently be provided in some cases.
Meanwhile, various attempts to improve a composition of the binder in order to solve the above-described problems of the sintered body have been proposed (Japanese National Patent Publication No. 60-014826 (Japanese Patent Laying-Open No. 56-069350, Patent Document 3), Japanese National Patent Publication No. 61-054857 (Japanese Patent Laying-Open No. 61-179847, Patent Document 4), Japanese National Patent Publication No. 61-054858 (Japanese Patent Laying-Open No. 61-179848, Patent Document 5), and Japanese Patent Laying-Open No. 05-287433 (Patent Document 6)). These proposals seem to achieve some effect in improving toughness, however, it has been difficult to drastically improve heat resistance, because the cubic boron nitride particles are separate from each other and the structure is discontinuous.
Meanwhile, a sintered body in which a continuous structure of cubic boron nitride particles is implemented by bonding the particles to each other has been known (U.S. Pat. No. 5,639,285 Specification (Patent Document 7)). In the sintered body, as the cubic boron nitride particles exhibit the continuous structure, an effect to improve heat resistance, that is, an effect to suppress increase in the temperature at the cutting edge, can be expected to some extent. On the other hand, as the binder is scattered discontinuously around the continuous structure of the cubic boron nitride particles, the sintered body is poor in chipping resistance or crater wear resistance, and consequently it is poor in toughness.
It appears that such a defect is mainly attributed to difference in coefficient of thermal expansion between cubic boron nitride and the binder contained in the sintered body. Specifically, the cause is estimated as follows. When the cutting tool including the sintered body is exposed to a high temperature during the cutting process, volume change occurs due to such temperature change (including not only temperature increase but also subsequent cooling). Here, the variation in the volume is considerably different between cubic boron nitride and the binder. Therefore, dissociation between cubic boron nitride and the binder occurs, and consequently, the binder or the like tends to separate from the sintered body.
Such a defect is particularly problematic in cutting a high-hardness material such as quenched steel, because exposure of the cutting edge of the cutting tool to a high temperature of 600° C. or higher is more likely. In addition, improvement in productivity has recently been demanded in a cutting process operation. With the increase in a cutting speed or feed rate, the temperature at the cutting edge is often raised to a temperature around 1000° C. Solution of the problem described above has thus been desired.
In addition, a cutting process of a part having a complicated shape has increasingly been demanded. In high-efficiency cutting such as interrupted cutting involved with more complicated shape, leaving and contact (catching) of the cutting edge of the cutting tool from/with a work material is repeated, and the cutting edge is rapidly cooled when the cutting edge leaves the work material. Accordingly, the sintered body contained in the cutting edge portion experiences sudden temperature change and stress change. Therefore, in high-efficiency process or high-speed interrupted cutting of the high-hardness steel that has recently been demanded, solution of the problems as pointed out above has particularly been desired.
Under the circumstances, it has been demanded to highly attain improvement in both heat resistance and toughness of the sintered body of the cubic boron nitride. Various developments of means for meeting such a demand have been attained, however, in these developments, study has been conducted on the premise that high strength is attained by firmly bonding cubic boron nitride to the binder contained in the sintered body. Therefore, on the premise that more advantageous bonding is obtained with a greater contact area between cubic boron nitride and the binder, at least one of cubic boron nitride and the binder has been implemented as a discontinuous structure instead of the continuous structure, in order to increase the contact area.
Patent Document 1: Japanese Patent Laying-Open No. 53-077811
Patent Document 2: Japanese Patent Laying-Open No. 10-182242
Patent Document 3: Japanese National Patent Publication No. 60-014826 (Japanese Patent Laying-Open No. 56-069350)
Patent Document 4: Japanese National Patent Publication No. 61-054857 (Japanese Patent Laying-Open No. 61-179847)
Patent Document 5: Japanese National Patent Publication No. 61-054858 (Japanese Patent Laying-Open No. 61-179848)
Patent Document 6: Japanese Patent Laying-Open No. 05-287433 Patent Document 7: U.S. Pat. No. 5,639,285 Specification